Optical encoders are used to monitor the motion of, for example, a shaft such as a crank shaft. Optical encoders can monitor the motion of a shaft in terms of position and/or number of revolutions of the shaft. They typically use a codewheel attached to the shaft to modulate light as the shaft and the codewheel rotate. The light is modulated as it passes through a track on the codewheel that includes a pattern of transparent and opaque sections. As the light is modulated in response to the rotation of the codewheel, a stream of electrical signals is generated from a photodetector array that receives the modulated light. The electrical signals are used to determine the position and/or number of revolutions of the shaft.
FIG. 1A depicts an example optical encoding system that includes a codewheel 10, a light source 12, and a photodetector array 14. The codewheel has a pattern of transparent and opaque sections 16 and 18 that alternatively pass and block light 20 from the light source. The optical encoding system is configured such that light passes through the transparent sections of the codewheel and is detected by the photodetector array that is located on the other side of the codewheel. Because light passes through the transparent sections of the codewheel, this type of optical encoding system is referred to as a “transmissive” optical encoding system. Transmissive optical encoders are able to generate output signals with good contrast between light and dark and as a result are able to operate at high speeds with relatively high resolution. Although transmissive optical encoding systems provide high quality outputs, the transmissive configuration requires that the light source and photodetector array be located on opposite sides of the codewheel, thereby putting limitations on the profile dimension of the encoding system.
Optical encoding systems that utilize reflection instead of transmission also exist and are referred to as “reflective” optical encoding systems. FIG. 1B depicts an example reflective optical encoding system that includes a codewheel 10 with a pattern of reflective and non-reflective sections 22 and 24 that alternatively reflect and absorb (or diffuse or pass etc.) light from a light source 12. The reflected light is then detected by a photodetector array 14. Because portions of the codewheel are reflective, the light source and photodetector array can be located on the same side of the codewheel, thereby allowing for a compact profile dimension. Although reflective optical encoding systems are conducive to a compact profile, they suffer from relatively low signal contrast, which restricts the speed and resolution of these encoding systems.